This invention relates generally to methods and apparatus useful for determining the composition of materials under study, and more particularly to methods and apparatus for analyzing and/or imaging specific materials in objects under study.
Because many known CT detection systems do not provide energy resolution, it is not possible to provide material characterization information for an object under study. For example, a highly attenuating material with low density can produce the same CT number in an image as a less attenuating material with high density. As a result, known computed tomographic (CT) images do not differentiate materials that have similar density but different atomic numbers, and images may look substantially uniform even though an object under study has variations in its material composition. In addition, beam-hardening artifacts, such as non-uniformity, shading, and streaking can result from the non-linear relationship between x-ray attenuation and path lengths for polychromatic x-ray beams in CT imaging systems. Also, known CT imaging systems do not provide quantitative image values. Instead, the same material at different locations can show different CT numbers.
At least one known dual energy decomposition algorithm is known that represents material-specific characteristics as a two-parameter basis set. Thus, by encoding each of these parameters individually, two separate images can be formed using a CT system. In at least one known system, a single slice image is acquired using a single slice CT detector system, using two different x-ray beam filters or two different x-ray tube voltages (kVp's). The different filters or voltages are used to obtain scan the same slice of an object. The two scans are not performed simultaneously, but instead are performed at slightly different times, e.g., sequentially. In another known system, energy sensitive scanning is performed by using an energy sensitive detector system such as a photon counting detector. In either case, the two energy dependent data sets are used with an appropriate material decomposition algorithm to produce two images, each representing one of the two basis materials.
In two basis material decomposition images produced by known imaging systems, the imaging value for each pixel in an image is equal to the material density for the corresponding basis material. Any material other than the two basis materials will show up in both images, with the image pixel value being proportional to the density of the non-basis material.
Using known two basis material decomposition algorithms, any material other than the two basis materials appears in both basis material images with an incorrect density. This contamination reduces the visibility of the basis materials in the images, and also results in density errors in quantification applications.